docs: def _iter_metadata_triples(cls, metadata_archive_path):
          """Yields all meta-data of Project Gutenberg texts contained in the
          catalog dump.
  
          """
          pg_rdf_regex = re.compile(r'pg\d+.rdf$')
          with closing(tarfile.open(metadata_archive_path)) as metadata_archive:
              for item in metadata_archive:
                  if pg_rdf_regex.search(item.name):
                      with disable_logging():
                          extracted = metadata_archive.extractfile(item)
                          graph = Graph().parse(extracted)
                      for fact in graph:
                          if cls._metadata_is_invalid(fact):
                              logging.info('skipping invalid triple %s', fact)
                          else:
                              yield fact